What should I do if I receive a 1099k from an online platform that I use for both business and personal payments?

From what you're saying, it doesn't sound like you'll need to report the transactions related to personal items sold for yourself, family and friends. 

When you're re-selling personal items, you only have to report the proceeds if you sold them for more than what you originally paid. 

The proceeds aren't taxable, but if you're audited, you'll need to be able to prove your cost.

Whatever transactions are related to business, you’ll need to report on Schedule C.  Please see the TurboTax Help article How do I enter a 1099-K in TurboTax Online? for guidance.

Also see the TurboTax Help article Do I have to report personal items that I sold? for more information.